1. Field of the Invention
The present invention relates to the removal of organics such as oils, including petroleum crude oil and products thereof, shale oils, oils from tar sands and the like, and organic contaminants, from underground accumulations of organics, such as natural subsurface formations and sites of hydrocarbon contamination. The invention is particularly useful for removal of residual organics, particularly constrained organics (those that can not be readily removed, e.g., residual organics) during enhanced oil recovery and soil remediation. The invention further relates to an in situ hydrous pyrolysis/oxidation process useful for partial in situ oxidation of the residual organics to generate surfactant molecules within subsurface water to enhance the mobility of the remaining bulk organic accumulations.
2. Description of Related Art
Subsurface formations that contain accumulations of organics, such as oils or contaminant hydrocarbons trapped therein, can be referred to as reservoirs. Removal of such organics from reservoirs by naturally occurring forces such as expanding high pressure gas and buoyant forces from encroaching water or gravity, is considered a primary recovery technique. Constrained organics are residual subsurface organic compounds retained in the subsurface after oil recovery or contaminant remediation techniques have been applied. These organics are normally in regions of relatively low permeability, or where such compounds remain tightly adsorbed onto surfaces of various mineral phases, or where moderate to low concentrations of the compounds remain behind as dissolved components in the groundwater phase. Such constrained organics are most often the difficult-to-remove residual compounds resulting from leakage or spills of organics, i.e., contaminant hydrocarbons or contaminants, or the difficult-to-produce naturally occurring oils, e.g., petroleum, shale oil, tar sands, bitumen, and the like.
In the former instance, underground fuel storage-tank leakage and industrial spills have posed a serious environmental problem. Fuel leaks contribute significantly to the contamination of groundwater by gasoline, aviation fuel, and other refined petroleum derivatives. Industry, such as electronic, chemical and chemical cleaning plants, are responsible for contamination of ground water with halogenated, typically chlorinated solvents. Many chlorinated hydrocarbons and components of fuels are of particular concern because they are confirmed or suspected carcinogens.
Many systems and methods have been developed to address the problems posed by such contaminated sites. Examples include systems for containment of the contaminants, pump-and-treat technology, methods for enhanced removal such as in situ dynamic underground stripping followed by ex situ treatments of contaminants, and methods for in situ treatments using various chemical and biological agents. Such systems or methods, however, do little for removing all contamination and cannot actually complete the remediation. The methods are unable to destroy or degrade the substantial residual amounts of hydrocarbon contaminants, i.e., constrained hydrocarbons, attached to the rocks, gravel, sand, clay or soil after the major decontamination efforts. A serious problem results because the remaining bulk phase organic contaminants continue to serve as slow release sources for sustained groundwater contamination. Capillary forces hold this free organic liquid tightly in the smaller pore spaces of the rock or soil, resulting in a xe2x80x9cresidual saturationxe2x80x9d of organic liquid, i.e., that which cannot be removed by pumping. This can amount to up to 20% of the liquid present. Thermal remediation methods address this residual contaminant principally by attempting to volatilize it and transport it in the vapor form, a process which is not effective for high-boiling point organics.
Current methods offer only incomplete remediation essentially because much of the subsurface contamination is deeply embedded into soils through diffusion and sorption. When there is free organic liquid present, its release to the aqueous phase may also be limited by solubility. These sorts of limitations are known as mass-transfer limitations. Many of the cleanup methods mentioned above would work if not for these mass-transfer limitations. Thermal methods overcome these mass-transfer limitations by viscosity reduction, accelerating the rates of diffusion and sorption/desorption and by increasing the solubilities and volatilities of the contaminant compounds. Although other oxidative methods have been proposed utilizing permanganate salts, Fenton""s reagent, ozone, or other oxidants, they too suffer from the mass-transfer limitations. In addition, they present problems stemming from the inability to mix the reagent with the contaminant in the subsurface.
Attempts to design permanent containment systems for underground contaminants are not practical as such systems need to be properly and continuously maintained and monitored for indefinite periods of time. These systems may hold the contaminants within the system, but they do not remove or degrade them. Consequently, when using this approach, the problem is never solved, but merely postponed. Any major natural disaster, such as an earthquake, may destroy these containment systems and the instant release of large amounts of constrained contaminant may be potentially extremely hazardous to the environment. Clearly, it would be advantageous to have a method available for the removal of these water and soil fuel hydrocarbon and chlorinated hydrocarbon contaminants which overcomes problems currently encountered with containment systems and in situ and ex situ treatments.
Several in situ methods for cleaning-up volatile organic compounds (VOC) involve the application of either heat alone or heat plus water and/or steam to mobilize volatile contaminants. This approach is essentially based on the physical properties of the VOCs. As the name implies, under appropriate conditions these contaminants volatilize. A good example of major efforts for fuel spill decontamination is a recently developed method for in situ dynamic underground stripping (DUS). The method, which is useful for removal of large amounts of volatile contaminants, is described in the Interim Progress Report, DOE publication UCRL-ID-109906 (1991), and in UCRL-1D-118187 (1994). During dynamic underground stripping, a targeted site is heated to vaporize the volatile contaminants. Once vaporized, the contaminants are removed from the spill site by vacuum extraction and treated ex situ. Dynamic underground stripping seems to be the best technique currently available to treat the large fuel spills. The lowest cost for treatment is associated with contaminant recovered as free-product liquid, due to its low total volume for handling. The dynamic underground stripping method alone is highly superior to conventional vacuum recovery. In combination with the current process of the invention, almost complete decontamination can be achieved in a very short time.
One of the major problems facing the remediation of volatile contaminants and solvents is the remaining low concentration of volatiles which, while volumetrically insignificant, can render water undrinkable. The difficulty in removing these residual contaminants (constrained organics), owing to the limitations posed by mass-transfer at low temperature, makes it nearly impossible to remove volatiles from most aquifers down to maximum contaminant levels of the drinking water standards. The cost of the process, and the time to accomplish it, are prohibitive and prevent remediation of low-level contamination using the mass-transfer limited methods.
Another trend in contaminant removal utilizes biological agents such as existing biota, bacteria, etc. For example, U.S. Pat. No. 5,279,740 describes a process for improved removal of contaminants from ground waters. The process utilizes simultaneous introduction of steam and specific nutrients effectively enhancing the growth of naturally occurring or added hydrocarbon degrading biota. The patent utilizes separate wells for adding the steam and nutrients and separate extraction wells for removal of extracted liquids and gas vapors containing the contaminants. As an ex situ method, it suffers from the problems enumerated above.
The solubility of organics in water tends to decrease as the total polarity of the organic decreases, making some of the most dangerous contaminant chemicals (e.g. trichioroethene, benzo(a)pyrene) essentially insoluble in water. Therefore they are very difficult to remove from contaminated soil by flushing with water, as in the pump-and-treat remediation methods. The use of thermal remediation methods, such as steam injection, can greatly increase the recovery of the more volatile organics but is still limited in the less soluble cases, such as the polycyclic aromatic hydrocarbons like benzo(a)pyrene.
One commonly used method of increasing the solubility of non-polar organics is to add surfactants, molecules that have polar groups and non-polar groups. This can effect direct co-solublization, where one end of the surfactant partitions into to the organic molecule(s) of interest, and the other partitions into the water, increasing the effective solubility of the organics. Often this mechanism is further enhanced by the formation of emulsions and spherical micelles, structures in which a hollow sphere of the surfactant molecules surrounds a core of the non-polar organic molecules. The polar end of the surfactant molecule points out of the sphere into the water, and the non-polar end points inward to the organic droplet. This principle is widely applied in creating cleaning agents, environmental cleanup processes, and enhanced oil recovery methods. In these applications surfactants are normally added to the system to create the micelles. In at least one instance, U.S. Pat. No. 3,036,631 describes an enhanced oil recovery method wherein organic acids, alcohols, ethers, aldehydes, ketones, etc., are produced below the surface and such compounds exhibit surface-tension-depressing properties. Mixed with injected caustic materials, as described in U.S. Pat. No. 2,288,857, such compounds produce emulsifying agents for petroleum oils.
The use of surfactants has met with limited success in the oil field and environmental remediation. Two difficulties arise; the chemical surfactants are expensive to produce, and it is difficult to get them into intimate contact with the organics, i.e., oil or contaminant molecules, underground. When these obstacles are overcome, the use of surfactants in both fields has greatly increased recovery. Surfactants are particularly effective at reducing the residual saturation of oil or organic liquid by reducing the surface tension between the water and oil. Torabzadeh and Handy, Society of Petroleum Engineers bulletin SPE 12689, 1984, show reductions from 32% to 25% in residual saturation of n-dodecane in Berea sandstone on addition of a common surfactant; upon increasing the temperature to 170 degrees C. the residual saturation was reduced to less than 3%. Because of the difficulty of simultaneously heating rock and mixing the surfactant with the oil, no huge benefit has been realized in oil production.
The present invention overcomes such difficulties by generating the surfactant in place (in situ), from the organic that is to be removed. The cost is much lower, injection of chemical is not required, the surfactant is created at exactly the location of the organic, and the surfactant is inherently well suited to mobilizing that product because it is derived from that product and has similar polarity and functional groups. Furthermore, in the case of utilization as a follow-on method subsequent to thermal treatment such as dynamic underground stripping, the new method takes advantage of the already existing injection-withdrawal wells and the persistently elevated underground temperatures for hydrous pyrolysis/partial oxidation affecting or enhancing the removal of constrained organics.
The present invention is a process for in situ hydrous pyrolysis/partial oxidation (HPPO) of constrained organics such as petroleum and petroleum products, including fuel hydrocarbons, polycyclic aromatic hydrocarbons, chlorinated hydrocarbons, and other volatile contaminants. The present process involves in situ partial oxidation of organics in water (normally liquid phase water such as groundwater and/or condensed steam) contained in subsurface soil or rock to produce intermediate oxygenated organic compounds, e.g., surfactants and precursors thereof, rather than completely oxidized compounds resulting from in situ hydrous pyrolysis/oxidation (HPO). In particular, the process involves thermal partial oxidation of organics including crude oils, petroleum products, including chlorinated or fuel hydrocarbons, petroleum distillates, polycyclic aromatic hydrocarbons, and other contaminants present in the soil and reservoir water, into oxygen-containing organic products of the oxidation, such as oxygen-containing functional groups (such as aldehyde, ketone, alcohol, or carboxylic acid functionalities) that are more soluble, but are also much more polar than the original forms, and therefore become surfactants. The process uses heat distributed through soils and water combined with oxygen or air or catalysts to produce the surfactants.
In this process, hydrous pyrolysis/partial oxidation is induced at an underground site containing subsurface constrained organics by introducing, under pressure, to the site either steam, oxygen (or air), a catalyst of the organic partial oxidation, or any combination thereof. The hydrous pyrolysis/partial oxidation proceeds, resulting in partial oxidation of the organics to the oxygen-containing functional groups that produce surfactants. The rate and degree of the partial oxidation is monitored to insure incomplete oxidation of the organics and the desired amount of surfactant formation.
The invention allows hydrous pyrolysis/partial oxidation of the aqueous mixture to produce surfactants that decrease the interfacial tension between the oil and water, and accordingly stabilizing oil-in-water emulsions. Subsequently, the oil/water emulsion can be removed from the underground formation.
Alternatively, the hydrous pyrolysis/partial oxidation of organics may be achieved by introduction of steam only to the groundwater (without addition of oxygen or a catalyst) and an oxidant of the target organics, where the oxidant is dissolved oxygen or air and/or a mineral present in the aquifer ground. The oxidant may include the dissolved oxygen already present in the water that contains the target organics, or mineral oxidants such as MnO2 or Fe2O3, which are already present naturally in soils and rocks or may be added as oxidation catalysts.
The process may use already raised temperatures in the underground remaining after in situ dynamic underground stripping method, and introduce only oxygen or a catalyst effective for partial oxidation of organics. The heating of the ground may also be accomplished by electrical resistance (joule) heating, radio frequency or microwave heating, or other electrical heating means.
The present invention, hydrous pyrolysis/partial oxidation, has been demonstrated successfully to partially oxidize organic compounds such as naphthalene, creosote compounds (e.g., as contained in pole tar), ethylbenzene, and methyl-tertbutyl ether (MTBE). Stoichiometric amounts of dissolved oxygen in the groundwater will lead to complete oxidation of the compounds of the constrained organics; however, in the present invention, less than such stoichiometric amounts of dissolved oxygen to organics is necessary in the groundwater for partial oxidation of the organics (as required under the given concentration conditions of the reservoir). The compounds of the organics are rapidly (days to weeks) converted to surfactants as a result of such partial oxidation at temperatures from about 40xc2x0 C.-350xc2x0 C. Nevertheless, it may be unnecessary to add additional oxygen in cases where a stoichiometrically sufficient amount of oxygen is already present in the organic-containing groundwater to produce surfactants. At lower temperatures (e.g., 90xc2x0 C.), partial oxidation proceeds at a slower rate and other, less completely oxidized compounds are included in the partially oxidized products effective as precursors for the desired surfactants.
The invention can be used to enhance the recovery of free organic liquids (non-aqueous phase liquids) during soil remediation processes, such as pump-and-treat remediation of contaminated ground water. The present method can apply to all organic contaminants susceptible to surfactant formation, and can be increasingly valuable with the less soluble contaminants such as coal tar, creosote, and manufactured gas plant waste. The present method has been effectively utilized to remove more than 500,000 lb of free-product of creosote from a creosote-contaminated subsurface site. Most of the recovered free-product creosote is in the form of the extremely stable oil/water emulsion formed by this method. The method can also assist the recovery of PCBs, pesticides, and other complex, low-solubility contaminants.
Recovery of heavy crude oil can be enhanced by the present method during steam flooding or other reservoir heating. Recovery of oil can occur as an oil-in-water emulsion . Enhanced recovery of oil by in situ surfactant formation can be enhanced from portions of a formation where retention is too great for oil to be produced by conventional steam flood applications, and can extend the productive life of an oil reservoir.